1. Field of the Invention
This invention relates generally to package systems and, more particularly, to a screw type closure application which incorporates an improved safety feature and enables improved productivity of the closure.
2. Description of the Prior Art
Screw cap or closure systems fall into two general categories. In the first class the caps have a continuous internal thread which cooperates with a corresponding external thread formed on the neck portion or finish of a container. In the second class, caps have a plurality of spaced lugs or have interrupted threads formed on the internal surface of a skirt wall, which cooperate with corresponding cams or interrupted thread segments formed on the external surface of the neck portion or finish of a container. The first class is so common that examples will not be cited here. The second class is illustrated in U.S. Pat. Nos. 1,612,449 and 4,202,462.
In addition to the two general classes discussed above there are hybrids which use spaced interrupted threads on a closure in combination with continuous threads on a neck portion of a container. U.S. Pat. No. 1,783,314 discloses a two-piece metal closure in which a flat strip of metal is bent into a tubular form with the ends interlocked to form a seam. A button top is then attached to the tube. Two turns of threads are interrupted at the seam to avoid deformation and disfiguration at the seam, and to fix the location of the seam with respect to the threads so that the seam will always be at the rear of the bottle. Although other embodiments illustrate the closure threads interrupted at more than one location, the thread segments are always vertically aligned with each other, as are the interruptions or spaces between the thread segments.
The trend today is toward closures which are molded from synthetic plastic materials. Such closures are in a single piece and typically consist of three main parts--a top wall, a skirt or cylindrical side wall depending from the top wall, and threads formed on the interior wall of the skirt. The top wall, in combination with any desired additional sealing means such as a liner or gasket, provides the necessary seal to protect the product in the container from the ambient environment and to retain internal pressures from carbonation and the like. The threads provide the capability of applying force to hold the closure in its sealing position on the container. The skirt acts as a link between the threads and the top wall.
For safety reasons it has been desirable to form more than one turn or 360 helical degrees of helical screw thread on the interior wall of the skirt of the closure, so that venting of internal pressures can occur before the threads in the closure and the threads on the neck of the container are disengaged. The length of such threads are as long as two turns or 720 helical degrees. Therefore, the user must perform at least two hand operations to remove the closure, permitting the internal pressure to vent. To aid in the venting process, some closures have provided vertically aligned gaps in the two or overlapping turns of thread, however the remaining thread portions are still vertically aligned with each other and suffer pulled-thread defects when molded closures are stripped from a mold.
In manufacturing a conventional unscrewing closure in a mold difficulties have been encountered. It has been a common practice to use an unscrewing device to remove a closure from the plug mold after the material has cured. However, the unscrewing adds time to the molding cycle and complexity to the molding equipment. In recent years, techniques have been developed for stripping the inside thread of a thermoplastic closure from the plug mold. Stripping is best used when the closure is designed with only one turn or 360 helical degrees of thread. As noted above, designs with more than one turn of thread tend to develop a defect known as pulled-threads in which the top of the threads are sheared by the force required to strip the closure from the plug mold.